The invention relates to a method of determining the extent or length of a keratotomy incision by determining a point of increased resistance against blade advancement of an RK knife as it makes the incision. The incision is stopped when a critical level of resistance is detected. The resulting incision length optimally corrects the patients vision, which is believed to occur as a result of minimizing strain within collagen fibrils of the corneal stroma, which strain is associated with refractive errors that produce a visual impairment requiring correction.
Ophthalmologists usually use "optical zone markers" to determine the length of a keratotomy incision. With the Fyodorov (Russian) technique, a radial RK incision typically begins approximately one millimeter from the outer edge of the cornea. The blade of an RK knife is moved radially inward toward an "optical zone mark" (which is a shallow indentation previously made in the epithelium by the surgeon around a central area of the cornea by pressing a stainless steel device known as a zone marker against the central cornea). An astigmatic keratotomy (AK) incision is performed by making the incision(s) tangential to the steepest radial curvature of the cornea in a linear or curvilinear fashion at a predetermined optical zone. The size of the optical zone marker usually is determined according to nomograms or empirical formulas based on age, sex, degree of correction needed, thickness, curvature, and/or diameter of the cornea. However, such nomograms or empirical formulas are based on averages for a large number of patients, and are not necessarily accurate for a particular patient if his or her eye has "non-average" material (i.e., tissue) properties.
Such nomograms typically are based on the response of a 30 year old caucasian having an intraocular pressure of 15 millimeters of mercury and little or no astigmatism. Typically, surgeons use "fudge factors" on data from such nomograms to develop an RK surgical plan for individuals differing from the typical 30 year old caucasian on which the nomograms are based. However, some patients may have exceptionally high corneal elasticity, or exceptionally low intraocular pressure, in which case even such "fudge factors" are likely to produce inaccurate results. When nomograms are used to design a surgical plan for radial keratotomy for such patients, there typically will be a large number of "undercorrections" of refractive errors. Since "overcorrections" are more difficult to surgically correct or "enhance" than undercorrections, most nomograms are weighted toward producing undercorrection. Therefore, later "enhancement" surgery is required in a high percentage of patients.
Mathematical predictions of the effect of proposed incisions on the cornea may be computed instead of (or in addition to) using nomograms to aid in preparing a surgical plan for radial keratotomy. The primary analytical tool used for this purpose is a "finite element analysis" (FEA) executed by means of a computer program. Instead of attempting to predict the effect of an incision on the entire surface of the cornea, such surface is subdivided into small "finite elements". After making certain assumptions about the biomechanical properties of a given cornea, both measured and nonmeasured data are applied to differential equations used in the FEA software, and the effect of the incisions on each of the finite elements "summed" across the entire cornea is determined. This helps predict the change of the corneal surface shape resulting from the proposed incisions.
FEA models of the cornea which have been described in the literature inherently depend on accurate determination of the biomechanical properties, some of which are not as yet accurately measurable. Furthermore, some assumptions regarding biomechanical properties of the cornea may not be accurate for an FEA model. Modeling of the effect of proposed incisions to develop a surgical plan for each patient by FEA models probably is not yet practical.
There is an unmet need for a technique for further optimizing lengths and number of keratotomy incisions based primarily on the material properties of the cornea and its behavior in immediate (acute) response to making of such incisions.
There also is an unmet need for a technique for minimizing strain in collagen fibrils of a cornea in such a way as to correct refractive impairment thereof.
There also is an unmet need for a way to reliably correct refractive errors with keratotomy on patients for whom techniques that rely on nomograms or empirical formulas do not produce accurate results.